1. Field of the Invention
The present invention relates to a ceramic electronic part, such as a laminated capacitor, and a method for producing the same. More specifically, it relates to a ceramic electronic part with an improved shape for the lateral edges of its internal electrodes and a method for producing the same.
2. Description of the Related Art
FIG. 6 is a cross-sectional view showing an embodiment of a conventional laminated capacitor. The laminated capacitor 51 has a ceramic sintered compact 52 comprising dielectric ceramics. A plurality of internal electrodes 53a to 53d are formed in the ceramic sintered compact 52 such that they are superimposed in the thickness direction via respective ceramic layers. The internal electrodes 53a, 53c extend to the end face 52a, and the internal electrodes 53b, 53d extend to the end face 52b. External electrodes 54, 55 cover the end faces 52a, 52b, respectively.
The ceramic sintered compact 52 to be used in the above-mentioned laminated capacitor 51 is produced by the process described below. A plurality of ceramic green sheets having internal electrodes 53a to 53d printed thereon are laminated together with an optional number of unprinted ceramic green sheets located on the upper and lower surfaces of the compact so as to obtain a laminated product. A conductive paste is used to form the internal electrodes. After the laminated product has been pressed in the thickness direction, it is baked.
Because the internal electrodes 53a to 53d are formed by baking the printed conductive paste, they have a substantially homogeneous thickness. The tip (one lateral edge) of the internal electrode 53a, that is, the part shown in the circle in FIG. 6, is shown in an enlarged view of FIG. 7A.
As apparent from FIG. 7A, the internal electrode 53a has a substantially homogeneous thickness to the tip. As shown in FIG. 7B, in some cases the tip of the internal electrode 53a may have a slight roundness.
However, due to the above-mentioned shape of the internal electrode, when the ceramic sintered compact 52 is formed, peel-off among the ceramic layers or the phenomenon called delamination sometimes occurs. This can be attributed to the fact that when the laminated product is pressed prior to baking, portions where the internal electrodes exist are compressed strongly in the thickness direction so as to have a higher density, whereas portions where the internal electrodes do not exist are insufficiently compressed. As a result, the density of the portions B, C and D shown in FIG. 7A before baking vary in the order B greater than D greater than C. This variation causes delamination or inter-layer peel-off in the sintered compact. In particular, in the case of an internal electrode with a thickness of 3 xcexcm or more (used for high reliability), this phenomena has been remarkable.
A method for alleviating the reduction in density in the part shown as C in FIG. 7A is disclosed in Japanese Unexamined Patent Publication No. 8-58259. This publication proposes a method of printing a conductive paste, using a special screen printing plate, wherein the thickness of the internal electrode in the vicinity of its outer rim is thinner than at its other parts. The cross-sectional shape of an internal electrode formed according to this method is shown in FIG. 8. In the vicinity of the outer rim of the internal electrode 56 shown in FIG. 8, the internal electrode 56 is thinner at part 56b as a result of the presence of step 56a. 
However, even in the sintered compact obtained by the method disclosed in the official gazette of Japanese Unexamined Patent Publication No. 8-58259, delamination and inter-layer peel-off is not satisfactorily prevented, and further improvement is desired.
An object of the present invention is to provide a ceramic electronic part such as a laminated capacitor, capable of effectively minimizing delamination and inter-layer peel-off phenomenon by improving the internal electrode shape, and a production method thereof.
A first aspect of the present invention is a ceramic electronic part comprising a ceramic sintered compact and a plurality of internal electrodes in the ceramic sintered compact, at least one lateral edge of at least one of the internal electrodes having a wedge-like cross-sectional shape, the length L of the wedge in the thickness t of the internal electrode at the base of the wedge satisfying the relationship L greater than 2t.
Since the lateral edge of the internal electrode is formed to have a wedge-like cross-sectional shape, with the length L of the wedge and the thickness of the internal electrode at the base of the wedge satisfying the relationship L greater than 2t, delamination or the inter-layer peel-off can be effectively restrained in the ceramic sintered compact. In particular, even in the case the thickness of the internal electrodes is increased to 3 xcexcm or more, since the lateral edge has the above-mentioned wedge-like shape, generation of delamination can be effectively restrained. Therefore, a ceramic electronic part with an excellent reliability can be provided as the production yield can be improved.
In a second aspect of the present invention, the internal electrodes and the ceramic sintered compact are selected such that the contraction ratio of the ceramic material forming the sintered compact by baking is larger than the contraction ratio of the material forming the internal electrodes. Since the materials are selected such that the contraction ratio of the ceramics by baking is larger than the contraction ratio of the internal electrodes, the lateral edges of the internal electrodes can securely have the above-mentioned wedge-like cross-sectional shape after baking.
In a third aspect of the present invention, the lateral edges of the internal electrodes are formed to have a wedge-like cross-sectional shape, with the length L of the wedge and the thickness of the internal electrode t at the base part of the wedge satisfies L greater than 2t. Since the lateral edges of the internal electrodes are formed to have a wedge-like cross-sectional shape, and to satisfy the relationship L greater than 2t, delamination can securely be prevented also at the side face of the ceramic sintered compact.
In a fourth aspect of the present invention, the ceramic sintered compact is made from dielectric ceramics, and the plurality of the internal electrodes are laminated via a ceramic sintered compact layer in the thickness direction of the ceramic sintered compact so as to provide a laminated capacitor. Since the ceramic sintered compact is made from dielectric ceramics, and the plurality of the internal electrodes are laminated via a ceramic sintered compact layer, a laminated capacitor with an excellent reliability without the risk of generating delamination in the ceramic sintered compact according to the present invention can be provided with a high yield.
In a fifth aspect of the present invention, the thickness of the internal electrodes is in the range of 3 to 20 xcexcm. With a thickness of 3 xcexcm or greater, the internal electrodes themselves and the connection between the internal electrodes and the external electrodes can be more reliable, and the current capacity can be larger compared with the case of less than 3 xcexcm. As to the upper limit of the internal electrode thickness, 20 xcexcm or less is preferable as mentioned above in view of having a small size.
According to the fifth aspect of a ceramic electronic part of the present invention, since the thickness of the internal electrodes is 3 xcexcm or greater, a laminated electronic part having a high reliability in the internal electrodes themselves and the connection between the internal electrodes and the external electrodes and a large current capacity can be obtained.
A sixth aspect of the present invention is a production method of a ceramic electronic part with a plurality of internal electrodes arranged in a ceramic sintered compact, comprising laminating a plurality of ceramic green sheets one on top of the other, at least some of the ceramic green sheets having conductive paste on a surface thereof such that the conductive paste is located between two adjacent ceramic green sheets, baking the laminated product to obtain the electronic part, the contraction ratio of the ceramic material forming the ceramic green sheets being greater than the contraction ratio of the conductive paste.
Since ceramics and a conductive paste with the contraction ratio of the ceramics by baking which is larger than the contraction ratio of the internal electrodes are used as the material of the ceramic green sheets and the conductive paste in obtaining a ceramic sintered compact after obtaining a laminated product by laminating a plurality of ceramic green sheets with a conductive paste for providing the internal electrodes printed and unprinted ceramic green sheets by baking the laminated product, a ceramic electronic part with the internal electrodes having a wedge-like cross-sectional shape according to the present invention can be provided easily and securely.
In particular, according to the production method of the fifth aspect, a ceramic electronic part according to the present invention can be provided in a process which is otherwise the same as the conventional production method by selecting the material for providing the internal electrodes and the conductive paste without the need of a jig such as a special screen printing plate, or the like. Therefore, a high reliable ceramic electronic part can be provided without the increase of the ceramic electronic part cost.